1. Field of the Invention
Aspects of the present invention relate to a switching mode power supply apparatus, and more particularly, to a switching mode power supply apparatus having a passive clamp circuit which suppresses a voltage stress of a main switch to control power supplied to a primary coil of a transformer.
2. Description of the Related Art
In conventional switching mode power supply apparatuses, such as flyback converters, forward converters, or the like, a voltage spike is generated at both ends of a main switch by an energy stored as a leakage inductance or a magnetizing inductance of a transformer during a switching operation so that a power loss is generated and an excessive voltage stress is applied to the main switch. A clamp circuit that forms a path for discharging the energy stored as the leakage inductance or the magnetizing inductance is used to suppress such voltage stress. By using the clamp circuit, a power loss of a switching device can be prevented from occurring by the energy stored as the leakage inductance or the magnetizing inductance.
Among various clamp circuits, passive clamp circuits comprising a clamping diode, a clamping capacitor, and a resistor have been widely used. Such passive clamp circuits are referred to as RCD (Resistor-Capacitor-Diode) clamp circuits. Such passive clamp circuits have simple structures and low manufacturing costs.
When the passive clamp circuits are used, a magnetizing energy is consumed by the resistor of the passive clamp circuit during a clamping operation. In a standby mode in which the power consumption of the switching mode power supply apparatus is greatly reduced, power consumed by the resistor of the passive clamp circuit takes a greatest portion of the power consumed in the entire switching mode power supply apparatus. As a result, there is a limitation in reducing standby power.